RI Carboyxlic acids and Derivatives Tutorial - Answers to Discussion Question

4 Suggested Answers to Carboxylic Acid & Derivatives Tutorial 1(a) Decreasing acid strength: ethanoic acid > phenol > ethanol In general, HA + H2O ⇌ H3O+ + A– The acidity of a compound depends on the relative stability of its conjugate base anion (A–). The more stable the conjugate base, the more acidic the compound will be. In the ethanoate ion (CH3CO2–), the negative charge on oxygen is dispersed over the two highly electronegative oxygen atoms resulting in two equivalent resonance structures. Hence, CH3CO2 ion is resonance-stabilised to a larger extent than C6H5O– ion. In the phenoxide ion (C6H5O–), the p-orbital containing the lone pair of electrons on the O atom overlaps with the -electron cloud of the benzene ring so that the negative charge on O delocalises into the benzene ring. The dispersal of negative charge stabilises C6H5O– ion but this resonance stabilisation is not as great as that in the CH3CO2 ion. In the ethoxide ion (CH3CH2O–), the electron-donating alkyl (ethyl) group intensifies the negative charge on O atom, which destabilises CH3CH2O– ion. Therefore, CH3CH2O– is the least stable. (b) Decreasing acid strength: trifluoroethanoic acid > trichloroethanoic acid > chloroethanoic acid > ethanoic acid In general, HA + H2O H 3O+ + A– The acidity of a compound depends on the relative stability of its conjugate base anion (A–). The more stable the conjugate base, the more acidic the compound will be. Compared to CH3COO ion, the CF3COO, CCl3COO and ClCH2COO ions are more stable due to the presence of electron-withdrawing F or Cl group(s) to disperse the negative charge and hence stabilising the anion. CCl3COO ion is more stable than ClCH2COO ion as it has 2 more electron-withdrawing Cl groups to disperse the negative charge. Since F is more electronegative than Cl, the F group is more electron-withdrawing than the Cl group and the negative charge is dispersed to a greater extent in CF3COO ion than in CCl3COO ion. Hence CF3COO ion is more stable than CCl3COO ion. 2 Decreasing ease of hydrolysis: C6H5COCl > C6H5CH2Cl > C6H5Cl The carbon of the acyl group in C6H5COCl has a higher + charge (or is more electron deficient) as it is bonded to two electronegative atoms (O and Cl). The carbon bonded to the chlorine atom in C6H5CH2Cl has lower + charge (or is less electron deficient) as it is bonded to only one electronegative atom (Cl). Hence, C6H5COCl can attract nucleophiles more easily and is more susceptible to nucleophilic attack as compared to C6H5CH2Cl. In addition, the carbon of the acyl group in C6H5COCl, being sp2 hybridised and trigonal planar, provides less steric hindrance during nucleophilic attack compared to the carbon bonded to the chlorine atom in C6H5CH2Cl, which is sp3 hybridised and tetrahedral. Thus, C6H5COCl undergoes hydrolysis with ease with water (a weak nucleophile) while C6H5CH2Cl requires a stronger nucleophile OH− und